There has recently been interest in improving the fuel efficiency of internal combustion engines. One route to this goal has been research toward development of engines with ceramic components. Ceramic components are useful because they are generally believed to be able to withstand higher operating temperatures than can customary metal parts. Modified engines which make use of higher operating temperatures can exhibit more efficient fuel use and are sometimes operated with reduced cooling requirements. As a result, however, there is a need for lubricants useful in such ceramic-containing engines which exhibit good high temperature properties such as oxidative and thermal stability. This is particularly true since the lubricant is sometimes used as a coolant fuel for selective engine components (e.g. cylinder heads and liners and pistons). Furthermore, lubrication of ceramic parts, including ceramic-coated parts, i.e. ceramic-ceramic and ceramic-metal interfaces, can be more demanding than lubrication of ordinary metal-metal interfaces. This is in part because of the higher temperatures encountered, but also because of the greater hardness of ceramics, compared to metal, results in increased pressure and stress at points of contact. Moreover, the chemical interaction of ceramics with lubricants and lubricant additives can be different in certain respects from the chemical interaction with metals. Accordingly, the lubrication of ceramic-containing engines, and in particular high temperature, low heat rejection ceramic-containing engines, presents a technical challenge.
PCT publication WO 91/13133, Sep. 5, 1991, discloses a high temperature functional fluid comprising a synthetic base oil, at least one phenolic compound, and at least one non-phenolic antioxidant. The synthetic base oil can be synthetic ester oils including those prepared from polyhydric alcohols and alkanoic acids, including fatty acids which contain from 5 to about 30 carbon atoms such as saturated straight chain fatty acids or the corresponding branched chain fatty acids or unsaturated fatty acids. The functional fluids are useful as lubricating compositions for lubricating engines operating at high temperatures such as high temperature, low heat rejection diesel engines.
U.S. Pat. No. 4,879,052, Mullin, Nov. 7, 1989, discloses improving friction and fuel consumption especially for an adiabatic diesel engine, by use of a lubricant comprising polyol ester and triaryl phosphate. The polyol ester is e.g. trimethylol-propane tri-isostearate or trimethylolpropane tripelargonate.